Kinetics of the Reactions of Chlorine Atoms with C2H4 (k1) and C2H2 (k2): A Determination of ΔHf,298° for C2H3

The rate constant ( k 1 ) for the reaction of Cl atoms with C 2 H 4 has been measured as a function of pressure (0.2−100 Torr) at 297 K using the relative rate technique in two reactors with either FTIR or GC analysis. The results of these and previous experiments (100−3000 Torr) can be described to within ±10% by a Troe expression with the limiting rate constants k 1 (∞) = (3.2 ± 0.15) × 10 -10 cm 3 molecule -1 s -1; and k 1 (0) = (1.42 ± 0.05) × 10 -29 cm 6 molecule -2 s -1 using F cent = 0.6. The stated uncertainties are statistical only, and the true uncertainties in the limiting rate constants must include uncertainty in F cent . Temperature-dependent (297−383 K) measurements of k 1 were carried out in the low-pressure regime (0.2−2 Torr) yielding the rate constant expression k 1 (0) = (1.7 ± 0.3) × 10 -29 ( T /298) -3.28 cm 3 molecule -1 s -1 . The rate constant, k 1b, for the abstraction channel of reaction 1 to form the vinyl radical was determined at 300, 343, and 383 K. This measurement of k 1b in combination with literature values of k - 1b allows a determination of the heat of formation of the vinyl radical by the third law method [Δ H f,298 °(C 2 H 3 ) = 70.6 ± 0.4 kcal/mol]. Using the same data with the second law yields Δ H f,298 °(C 2 H 3 ) = 69.6 ± 1.6 kcal/mol. These measurements agree satisfactorily with a recent negative-ion photoelectron spectroscopy determination of this quantity (71.6 ± 0.8 kcal mol -1 ). The pressure dependence of the rate constant ( k 2 ) for the reaction of Cl with C 2 H 2 was determined over the range 0.3−700 Torr. The results of these and previous experiments (100−6000 Torr) can be described to within ±10% by a Troe expression with the limiting rate constants k 2 (∞) = (2.0 ± 0.1) × 10 -10 cm 3 molecule -1 s -1 and k 1 (0) = (6.1 ± 0.2) × 10 -30 cm 6 molecule -2 s -1 using F cent = 0.6.